1. Field of the Invention
This invention relates to a crystal growth method of an oxide and a multi-layered structure of oxides particularly suitable for application to oxide electronics developed on silicon substrates.
2. Description of the Related Art
Silicon oxide (SiO2) films made by thermal oxidation of silicon (Si) have been exclusively used as gate insulating films of MOS-FET (metal-oxide-semiconductor FET) because of their high electric insulating ability, low interface state density, easiness to process, thermal stability, and other advantages. These SiO2 films made by thermal oxidation for use as gate insulating films, however, have low specific dielectric constants (xcex5r xcx9c3.8) and must be formed very thin on Si substrates. Along with the progress toward thinner gate insulating films, short channels and other requirements to meet the demand for integration, various problems arose such as dielectric break-down of gate insulating films and pinch-off of channels caused by influences from source-drain voltages (short channel effect), and gate insulating films will soon come to the limit in terms of their materials. Under the circumstances, the need for new gate insulating films having high dielectric constants is being advocated as a technical subject of MOS-FET of the sub 0.1 micron generation, in addition to a further progress of lithographic technologies, needless to say (for example, (1) MTL VLSI Seminar (Massachusetts Institute of Technology)). If EL gate insulating film can be made by using a material of a high dielectric constant, it will need not be so thin. Therefore, gate leakage will be prevented, and short-channel effects will be prevented as well.
On the other hand, researches on ferroelectric non-volatile memories (FeRAM) have come to be active (for example, (2) Appl. Phys. Lett., 48(1986)1439, (3) IEDM Tech. Dig., (1987)850, (4) IEEE J. Solid State Circuits, 23(1988)1171, (5) 1988 IEEE Int. Solid-State Circuits Conf. (ISSCC88), (6) Digest of Technical Papers, THAM 10.6 (1988)130, and (7) Oyo Butsuri, 62(1993)1212). Among these ferroelectric non-volatile memories, what is considered to be closest to practical use is a ferroelectric non-volatile memory of a quasi-DRAM structure (using two-transistors and two-capacitors type memory cells, or one-transistor and one-capacitor type memory cells). This structure is advantageous in making it easier to prevent interference with the Si process because CMOS process and ferroelectric capacitor process can be separated by using an inter-layer insulating film. However, the structure of this ferroelectric non-volatile memory does not meet the use of a scaling law of a Si device. Therefore, as microminiaturization progresses, it is necessary to employ a more complex structure or use a material with a larger value of residual polarization in order to ensure a certain amount of charge storage in the capacitor. On the other hand, there are many research institutes tackling with the study of ferroelectric non-volatile memories using MFS (metal-ferroelectrics-semiconductor)-FET memory cells, including MFMIS (metal-ferroelectrics-metal-insulator-semiconductor)-FET memory cells, FCG (ferroelectric capacitor gate) memory cells, and so forth), which constitute two major subjects together with those of a quasi-DRAM structure mentioned above. The latter type ferroelectric non-volatile memories match with the scaling low, and merely need a very small value of residual polarization (about xcx9c0.1 xcexcC/cm2). Additionally, since they need only one transistor for storage and hence contribute to a decrease of the cell size, they are advantageous for high integration. Furthermore, since they are of a nondestructive readout type, they are more advantageous also against fatigue, which might be an essential problem of ferroelectric materials, than destructive readout type memory cells with two transistors and two capacitors, or one transistor and one capacitor, and are also available for high-speed operations. Because these excellent properties are expected, MFS-FET ferroelectric non-volatile memories are now recognized as ultimate memories ((8) Appl. Surf. Sci. 113/114(1997)656).
What is preventing practical use of these MFS-FET ferroelectric non-volatile memories is problems with their manufacturing process. It is extremely difficult to grow ferroelectric materials directly on Si substrates. Therefore, growth of buffer layers of insulating materials on Si substrates is recognized as one of most important technologies. In case of a MFIS (metal-ferroelectrics-insulator-semiconductor)-FET which is one of MFS-FET ferroelectric non-volatile memories, gate voltage is distributed to an insulating layer as well, and this invites the drawback that the write voltage is high. To prevent it, the insulating layer must be one with a high dielectric constant. On the other hand, material properties required as a ferroelectric material used here are a low dielectric constant, appropriate value of residual polarization (typically around xcx9c0.1 xcexcC/cm2, although depending upon the device design), and most seriously, good squareness ratio. Additionally, for the purpose of realizing a better interface, one of important conditions is that these materials can be grown at low temperatures. Thus, choice and development of materials is required from the standpoint different from that of the one-transistor and one-capacitor type. There are a lot of research reports on MFIS-FET ferroelectric non-volatile memories. However, because of insufficient interface properties, there is almost no reports about practically usable ones including the requirement for retention (charge retaining property).
On the other hand, it is greatly significant to introduce oxide materials other than SiO2 into the semiconductor industry. High-temperature superconductive materials discovered in 1986 ((9) Z. Phys. B., 64, 189-193(1986)), needless to say, and oxide materials especially having perovskite or related structures have very important physical properties for semiconductor devices, such as ferroelectricity, high dielectric constant, superconductivity, colossal magnetoresistance, and so forth ((10) Mater. Sci. Eng., B41(1996)166, and (11) J. Ceram. Soc. Japan, Int. Ed., 103(1995)1088). For example, among ferroelectric materials of capacitors for ferroelectric non-volatile memories mentioned above, zirconium titanate (PZT) having a large value of spontaneous polarization and a low process temperature (for example, (12) J. Appl. Phys. 70, 382-388(1991)) and bismuth strontium tantalate (Bi2SrTa2O9 ((13) Nature, 374(1995)627, (14) Appl. Phys. Lett., 66(1995)221, (15) Mater. Sci. Eng., B32(1995)75, (16) Mater. Sci. Eng., B32(1995)83, (17) Appl. Phys. Lett., 67(1995)572, (18) J. Appl. Phys., 78(1995)5073, (19) Appl. Phys. Lett., 68(1996)566, (20) Appl. Phys. Lett., 68(1996)690, and (21) International Laid-Open Publication WO93/12542) are the twin greatest materials. Furthermore, including the discovery of colossal magnetoresistance materials (CMR materials) in the group of Mn oxides, which are variable in resistivity over some digits under application of a magnetic field ((22) Phys. Rev. Lett. 74(1995)5108), great interest has come to be attracted to how high potential capacities these oxide materials have ((23) Mater. Sci. Eng., B41(1996)166, and (24) J. Ceram. Soc. Japan, Int. Ed., 103(1995)1088), and technologies for making thin oxide films have been developed remarkably in these ten years or so.
If oxide materials having these very high functional physical properties can be developed on Si which is the basis of the semiconductor industry, these materials will get a high marketability. However, because of difficulties between these functional oxide materials and Si, such as mutual thermal diffusion and differences in thermal expansion coefficient, it is usually difficult to directly grow these functional oxide materials on Si.
As discussed above, almost all of these functional oxide materials have structure based on a perovskite structure. Many of them, such as yttrium-system superconductive materials having critical temperatures beyond the liquid nitrogen temperature and Bi2SrTa2O9 mentioned above, have structures called layered perovskite having a very large anisotropy. In these layered perovskite structured oxides, superconductive current paths, polarization axes, etc. are limited to specific directions, and also in case of simple perovskite structured oxides, there are many in which polarization axes are limited to specific directions like that in PZT. Therefore, when they are used to make devices, it is important to specifically orient oxides, or more preferably, epitaxially grow them relative to bases, in order to draw out their maximum properties.
Ceria (cerium dioxide: CeO2) having a fluorite structure is one of candidates for materials of gate insulating films with high dielectric constants for the sub 0.1 micron generation to substitute for SiO2 films by thermal oxidation because of its thermal stability, high specific dielectric constant (xcex5r xcx9c26) and very good lattice matching with Si substrates (misfit: about 0.35%), and it is considered to be one of most ideal buffer layer materials for epitaxially growing perovskite-related oxides on Si substrates. Actually, researches are being made on epitaxial growth of ceria on Si substrates, but most of them are directed to CeO2(111)/Si(111) structures easy for atomic close-packed growth (for example, (25) Jpn. J. Appl. Phys. 34(1995), L688, and (26) Japanese Patent Laid-Open Publication No. hei 7-25698). With regard to Si(001) substrates which permit a channel to be formed in the Si less than 110 greater than  orientation having the largest mobility and are being used exclusively as MOS-FET substrates, it has been believed that CeO2(110) having an antiphase domain epitaxially grows, reflecting the dimer structure by the surface reconstructed structure of Si(001)-2xc3x971, 1xc3x972. Further, regarding the growth temperature, it is considered that a temperature around 800xc2x0 C. at which no SiO2 film is formed on the Si surface in a high vacuum ((27) J. Vac. Sci. Technol. A13(1995)772) is the lower limit of epitaxial temperature ((28) Jpn. J. Appl. Phys. 33(1994), 5219, (29) Appl. Phys. Lett. 56 (1990), 1332, (30) Appl. Phys. Lett. 59(1991), 3604, (31) Physica C 192 (1992)154, (32) Jpn. J. Appl. Phys. 36(1997), 5253, and (33) Japanese Patent Laid-Open Publication No. hei 9-64206). Furthermore, under the belief that the growth temperature must be lowered to obtain a steep interface, electron beam assisted epitaxial growth, for example, is under trial. However, the lower limit of epitaxial growth temperature heretofore reported is 710xc2x0 C. ((34) 1998 spring symposium of Oyo Butsuri Gakkai, Presentation No. 28p-PA-1).
Researches are being made also on yttria (yttrium oxide: Y2O3) having a C-rare-earth structure (bixbyite) because it has material properties similar to ceria. However, it involves the same problems as ceria, and in usual, Y2O3(110) epitaxially grows on Si(001) substrates ((35) Appl. Phys. Lett. 71(1997), 903).
On the other hand, there is an example in which a perovskite oxide epitaxially grows on ceria (or yttria) (for example, (36) Appl. Phys. Lett. 68(1996)553). Therefore, if ceria can be controlled in orientation, a device making use of the characteristics of functional oxides will be realized. Although CeO2(110)/Si(001) is not always the most desirable growth orientation, it is possible that CeO2(110)/Si(001) structure is acceptable or rather desirable, depending on the anisotropy of a functional oxide to be epitaxially grown thereon and a device design required.
Under these circumstances, technologies for epitaxially growing CeO2(110), which is one of important orientations, on Si(001) substrates most important for practical application at a temperature lower than 710xc2x0 C. which is the lowest temperature heretofore reported.
It is therefore an object of the invention to provide a crystal growth method of oxides capable of epitaxially growing cerium oxide, yttrium oxide, and rare earth oxides having crystal structures similar to them in the (110) orientation on (001)-oriented silicon substrates at a growth temperature lower than conventional ones to realize an epitaxial rare earth oxide (110)/silicon (001) structure; and a multi-layered structure of oxides.
The Inventors made researches toward resolution of the above-discussed problems involved in the conventional technologies. Their outline is explained below. In the explanation made below, growth of cerium oxide is taken as a typical example.
To attain the object of the invention, it is important that the surface of a silicon substrate has steps and terraces at least once. This can be realized by the method proposed by Ishizaka and Shiraki et al ((37) J. Electrochem. Soc., 133(1986)666), for example, or other similar methods. The growth apparatus is preferably one of molecular beam epitaxy (MBE) apparatus, laser ablation apparatus, reactive vacuum evaporation apparatus, and so forth, excellent in surface controllability and permitting observation of the surface by reflection high energy electron diffraction (RHEED). Basically, however, any apparatus is acceptable provided it can control the pressure, temperature, and so on. When growing cerium dioxide (CeO2), cerium dioxide itself is used as the source material of cerium in most cases. However, since oxygen with a low vapor pressure, selectively volatilizes, it is desirable to use cerium metal in order to control a low oxygen partial pressure. But oxide source materials are considered usable if the low vacuum can be maintained by additionally using a getter pump, for example for evacuation of the growth chamber, for example. Cerium metal has a high melting point and a low vapor pressure, and it is desirable to vaporize it by using a high-temperature Knudsen cell, electron beam vapor deposition, excimer laser, or the like. Additionally, for controlling interaction of these gases or metal elements, etc. with the highly active surface of the silicon substrate, it is important to employ a low-temperature process.
The Inventors made researches on conditions for epitaxial growth of CeO2(110) on silicon (001) substrates from various standpoints, including a decrease of the growth temperature to control rates of generating cerium silicide, silicon oxide, for example. In conclusion, it is essentially important to prepare the surface of the silicon (001) substrate in form of a dimer structure of 2xc3x971 and 1xc3x972 surface reconstruction before the growth, and it is also important how the source material is supplied during the growth. Regarding the latter subject, method of supply the source material, it is important, more specifically, to first start the supply of oxidic gas like oxygen onto the silicon (001) substrate surface and subsequently start the supply of the source material of Ce. Its reason has not been clarified yet. However, it is presumed that the surface of the silicon (001) substrate is covered by molecules or atoms of oxidic gas like oxygen before the supply of the source material of Ce is started, and this promotes (110) epitaxial growth of CeO2.
In addition to the above-mentioned matters, for epitaxial growth of CeO2(110), it is important to appropriately select the ratio of the supply amount of oxidic gas relative to the supply amount of the source material of Ce under a certain growth temperature, or the partial pressure of the oxidic gas or its supply amount when the supply amount of the source material of Ce is constant. FIG. 1 shows relationship between growth temperature T and ratio O/Ce of the supply amount of oxidic gas relative to the supply amount of the source material of Ce when the latter is constant. For convenience, shown on the ordinate of FIG. 1 are values of O2 flow rate [sccm] instead of O/Ce. It is known from FIG. 1 that there is a certain restriction in the region where CeO2(110) can epitaxially grow. Also known therefrom is that, as the general tendency, the O2 flow amount has to be increased as the growth temperature increases.
The Inventors reached the present invention after making investigation from various viewpoints, in addition to the above-explained researches and knowledge.
According to the first aspect of the invention, there is provided a crystal growth method of an oxide comprising the steps of:
processing a surface of a (001)-oriented silicon substrate into a dimer structure by 2xc3x971 and 1xc3x972 surface reconstruction; and
epitaxially growing a rare earth oxide of a cubic system or a tetragonal system in the (110) orientation on the silicon substrate in an atmosphere containing an oxidic gas by using a source material made up of at least one kind of rare earth element.
According to the second aspect of the invention, there is provided a crystal growth method of an oxide comprising the steps of:
processing a surface of a (001)-oriented silicon substrate into a dimer structure by 2xc3x971, 1xc3x972 surface reconstruction; and
epitaxially growing a rare earth oxide of a cubic system or tetragonal system in the (110) orientation on the silicon substrate by using a source material containing at least one kind of rare earth element in an atmosphere containing an oxidic gas,
when the rare earth oxide is epitaxially grown, the source material containing at least one kind of rare earth element being supplied after the supply of the oxidic gas onto the surface of the silicon substrate is started.
According to the third aspect of the invention, there is provided a crystal growth method of an oxide comprising the steps of:
processing a surface of a (001)-oriented silicon substrate into a dimer structure by 2xc3x971, 1xc3x972 surface reconstruction; and
epitaxially growing a rare earth oxide of a cubic system or tetragonal system in the (110) orientation on the silicon substrate at a growth temperature lower than 710xc2x0 C.
In the third aspect of the invention, the rare earth oxide is epitaxially grown typically at a growth temperature lower than 300xc2x0 C. and preferably at a growth temperature not higher than 100xc2x0 C.
According to the fourth aspect of the invention, there is provided a multi-layered structure of oxides comprising:
a (001)-oriented silicon substrate;
a CeO2 film grown on the silicon substrate at a first growth temperature; and
a (110)-oriented CeO2 film epitaxially grown on the CeO2 film at a second growth temperature higher than the first growth temperature.
According to the fifth aspect of the invention, there is provided a multi-layered structure of oxides comprising:
a (001)-oriented silicon substrate;
a SiOx film on the silicon substrate;
a CeO2 film grown on the SiOx film at a first growth temperature; and
a (110)-oriented CeO2 film epitaxially grown on the CeO2 film at a second growth temperature higher than the first growth temperature.
In the fifth aspect of the invention, x of the SiOx film is typically in the range of 0 less than xxe2x89xa63 and normally in the range of 1xe2x89xa6xxe2x89xa62.
In the fourth and fifth aspects of the invention, the first growth temperature may be selected arbitrarily in the range of the room temperature to 700xc2x0 C., for example.
Oxides multi-layered structures according to the fourth and fifth aspects of the invention can be illustrated as shown in FIGS. 2 and 3, respectively.
In the present invention, the (001)-oriented silicon substrate intends to involve a silicon substrate offset from the (001) orientation within a range which can be regarded to be substantially equivalent to the (001) orientation.
In the present invention, the rare earth oxide is any one of oxides of rare earth elements like cerium (Ce), yttrium (Y), and so forth, and may be an oxide of one kind of rare earth elements, or two or more kinds of rare earth elements. When the rare earth oxide is expressed as ReOz (where Re is one or more rare earth elements), 0 less than zxe2x89xa63 is normally satisfied. Although the rare earth oxide is a cubic system or a tetragonal system, the cubic or tetragonal system includes those slightly distorted within a range which can be regarded as the cubic or tetragonal system substantially. The rare earth oxide typically takes a fluorite structure (CeO2 structure) or a C-rare-earth structure (Y2O3 structure; bixbyite structure).
In the present invention, for the purpose of epitaxially growing the rare earth oxide in the (110) orientation more reliably, a source material containing at least one kind of rare earth element is supplied preferably after the oxidic gas is supplied onto the surface of the silicon substrate in the process of epitaxial growth of the rare earth oxide. The source material containing at least one kind of rare earth element may be either one made up of at least one kind of rare earth element or one made up of a rare earth oxide, for example. However, especially at high temperatures around 700xc2x0 C., since surface oxidation will start at the moment of the supply of the oxidic gas onto the substrate surface, also the supply of the rare earth element has to be started immediately. In appropriate cases, a very small amount of rare earth elements, e.g. in the order of EL monolayer or sub mono layer, may exists on the surface of the silicon substrate before the supply of the oxidic gas as it invites no trouble.
In the present invention, a further step of annealing may be provided to anneal the rare earth oxide in vacuum of a pressure not higher than 1xc3x9710xe2x88x926 Torr at a temperature not lower than the growth temperature of the rare earth oxide after epitaxially growing the rare earth oxide. After the rare earth oxide is epitaxially grown, another step of homoepitaxially growing a rare earth oxide on the former rare earth oxide at a growth temperature higher than that of the former rare earth oxide may be provided. After the rare earth oxide is epitaxially grown, another step of epitaxially growing a functional oxide on the rare earth oxide may be provided.
In the present invention, depending upon the growth conditions, a silicon oxide film or defective layer not thicker than 5 nm may be formed along the interface between the silicon substrate and the rare earth oxide after growth of the rare earth oxide.
In the present invention, the functional oxide typically has a perovskite structure or a layered perovskite structure. Essentially, any can be the functional oxide, such as, for example, ferroelectric material, superconductive material, pyroelectric material and piezoelectric material.
In the present invention, the (110)-oriented rare earth oxide by epitaxial growth is used as, for example, a gate insulating film of MIS-FET or a device separating insulating film, and can be used as a buffer layer as well when it is difficult to grow a functional oxide directly on a silicon substrate.
According to the invention having the above-summarized structure, by processing the surface of a (001)-oriented silicon substrate into a dimer structure by 2xc3x971, 1xc3x972 surface reconstruction and preferably supplying a source material containing at least one kind of rare earth element after starting the supply of an oxidic gas onto the surface of the silicon substrate in the process of epitaxial growth of a rare earth oxide, the rare earth oxide can be epitaxially grown excellently in the (110) orientation on the (001) oriented silicon substrate at a growth temperature lower than conventional ones.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.